Variable displacement pump having pressure compensator control method

ABSTRACT

A variable displacement pump of the type having a plurality of rotatable, axially-aligned pistons guided by a pivotal swash plate for enabling adjustment of displacement and also having a variable cutoff pressure compensator control means whereby the swash plate is variably shifted to its minimum displacement position so as to achieve a variable cutoff of pump flow. The pressure compensator control means also shifts the swash plate to its minimum displacement position when a predetermined maximum pressure is reached or when the pump load is in a neutral condition, thereby curtailing heat generation and horsepower loss.

United States Patent Hein et a1.

[ Aug. 20, 1974 VARIABLE DISPLACEMENT PUMP 3,221,660 12/1965 DAmato 417/222 V NG PRESSURE COMPENSATOR 3,234,889 2/1966 gpoper et a1. 492/122 3,266,434 8/1966 cAlvay 17/222 CONTROL METHOD 3,635,021 1/1972 McMillen et a1. 417/222 [75] Inventors: Allyn J. l-lein, Joliet; William B.

Norick, D p; f'l Primary ExaminerWilliam L. Freeh New Lenox; Gllbel' Trlbley, Jollet, Assistant ExaminerG. P LaPointe of Attorney, Agent, or Firm-Phillips, Moore, [73] Assignee: Caterpillar Tractor Co., Peoria, I11. welssenberger Lemplo & Strabala [22] Filed: June 25, 1973 [57] ABSTRACT [211 App]' 3736l2 A variable displacement pump of the type having a Related US, Application Dat plurality of rotatable, axially-aligned pistons guided by [63] Continuation of Ser. No. 157,535, June 28, 1971. a pivotal swash plate for enabling adjustment of displacement and also having a variable cutoff pressure 52] US. (:1. 417/217 cmpensatr cmmo means whereby the swash Plate 51 1111.01. F04b 21/00 is variably Shifted to its minimum placement posi- [58] Field at Search 91/499 505 506 12.1 as achieve a variable Pump 91/125 417/217 6 The pressure compensator control means also shifts the swash plate to its minimum displacement position [56] References Cited when a predetermined maximum pressure is reached UNITED STATES PATENTS or when the pump load is in a neutral condition, thereby curtailing heat generation and horsepower 2,291,424 7/1942 Wichorek 417/239 km 2,483,343 9/1949 Jeffrey 91/20 2,696,189 12/1954 Born et a1. 91/505 10 Claims, 6 Drawing Figures l "138 \13 r 9 22 b t "11152 1Z6 174 72 I g 1 6O 196 as 11 -12?! 1 28 I 76 78 1 1 26 A8 1 j u l -70 1;; 1 34 i I 50 M 52 j:

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smear 4 v 3 644 a i I I30 VENToRs ALLYN J HE WILLIAM B NOR'CK WALTER Z. RUS GILBERT TR|B J A l v.

ATTORNEYS PAIENTEB 3.830.594 sum 3 0F 4 'INVENTORS ALLYN LI. HEIN WILLIAM B. NORICK WALTER z. RUSEFF BY GILBERT TRIBLEY PAIfi-miumszmsu slmuai INVENTORS J. HEIN NORICK RUSEFF ALLYN WILLIAM B WALTER Z. GILBERT TRIBL EY VARIABLE DISPLACEMENT PUMP HAVING PRESSURE COMPENSATOR CONTROL METHOD This is a continuation, of Ser. No. 157,535, filed June 28, 1971.

CROSS-REFERENCE TO RELATED APPLICATION Reference is made to copending application Ser. No. 157,157, filed June 28, 1971, and now US. Pat. No. 3,738,779, issued June 12, 1973, assigned to the assignee of this invention, wherein is disclosed a similar device having sharp-cutoff-pressure compensator control means as opposed to the variable-cutoff-pressure compensator control means of the instant invention.

BRIEF SUMMARY This invention is directed to variable displacement pump of the type having a plurality of rotatable, axiallyaligned pistons guided by a nonrotatable, tiltable swash plate which enables adjustment of pump displacement by the operation of pressure compensator control means.

Currently, constant displacement, as opposed to variable displacement, pumps are widely used as a source of fluid pressure to power hydraulic circuits in many applications. For example, constant displacement pumps are used to power hydraulic implement circuits on heavy construction equipment such as tractors, etc. In this application there is no provision for returning pump flow to tank when the implement control valves are in neutral position unless an open-center system is used. However, with this type of system, an additional amount of engine horsepower is needed to drive the constant delivery pump at maximum-relief valve pressure even when no actuating fluid is demanded by the system. This, in turn, results in an undesirable, excessive amount of heat generation in the hydraulic system.

In addition, even if a pressure compensator control means such as disclosed in the cross-referenced application is used, actuation of more than one implement circuit at high pressure could stall the engine if some means were not provided to reduce the horsepower de manded during such operation. An exemplary application might be with regard to a backhoe where several operations are commonly conducted simultaneously.

Also, separate and apart from the problem of wasted engine horsepower and undesirable heat generation resulting from operation of the pump in the neutral, or no-load condition, there is the problem of cooling system overloading caused by continued operation of the pump at the maximum displacement position even after a predetermined maximum pressure is reached. This requires corrective action in the form of turning the pump to its minimum displacement position upon attaining the predetermined maximum pressure.

It is to a solution of these and other problems that the invention of this disclosure is directed.

It is, therefore, an object of this invention to provide a variable displacement pump having pressure compensator control means operable to control pump displacement in response to system pressure.

It is also an object of this invention to provide a vari-' able displacement pump having pressure compensator control means which functions to save horsepower and prevent excessive heat generation and stalling of the driving engine when one or more hydraulic implement circuits are actuated.

It is a further object of this invention to provide a variable displacement pump having a variable pressure compensator control means which functions to variably cutoff pump flow by variably adjusting pump displacement in order to limit horsepower loss and prevent excessive heat generation.

A still further object of this invention is to provide a variable pump having pressure compensator control means to variably cut off pump flow in response to pressure until a maximum pressure condition is reached, at which time sharp cutoff of pump flow is accomplished in order that system pressure will not exceed a preset maximum value.

It is a still further object of this invention to provide variable displacement pump having pressure compensator control means to variably change the pump displacement from a maximum in response to pressure and to immediately shift the displacement to a minimum when the system pressure reaches a predetermined maximum value, thereby limiting horsepower loss and preventing excessive heat generation.

It is a still further object of this invention to provide a variable displacement pump having pressure compensator control means utilizing pilot operation for controlling pump displacement whereby more stability and less hunting and overshoot are encountered when a change in displacement is required.

his a still further object of this invention to provide a variable displacement pump having pressure compensator control means in the form of interchangeable capsules whereby pump flow direction may be changed by merely interchanging capsules.

It is yet another object of this invention to provide a variable displacement pump having pressure compensator control means operable to provide variable cutoff of pump flow in response to system pressure as well as sharp cutoff of pump flow when the hydraulic implement circuits serviced by the pump are in a neutral condition or the system pressure reaches a predetermined maximum value.

It is yet another object of this invention to provide a variable displacement pump in which pump displacement may be changed by means of an external pressure source.

It is still another object of this invention to provide a variable displacement pump in which pump displacement may be changed by means of external means operatively connected to the pump by mechanical linkage means.

Other objects and advantages of the present invention will become apparent from the following description and claims as illustrated in the accompanying drawings which, by way of illustration only, show preferred embodiments of the present invention and the principles of operation thereof. It is to be understood that the scope of the invention is not to be limited thereto, but is to be determined by the scope of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top plan view of a hydraulic, variable displacement pump having hydraulic pressure compensator control means installed therein together with attendant hydraulic circuitry;

FIG. 2 is a vertical, cross-sectional view of the variable displacement pump taken along line IIII of FIG. 1, and illustrating in detail the mechanism for changing the displacement of the pump;

FIG. 3 is a partial vertical cross-sectional view of a capsule assembly taken along line III-III of FIG. 1;

FIG. 4 is a graphical illustration of the pump flow/- pressure characteristics of the variable-flow displacement pump of the subject invention;

FIG. 5 is a partial, vertical, cross-sectional view of the variable displacement pump along with attendant hydraulic circuitry illustrating a second embodiment of the pump internal mechanism; and

FIG. 6 is a similar view illustrating yet another embodiment of the pumps internal mechanism and attendant mechanical and hydraulic structures.

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIG. 1, there is shown generally at 10 a top view of a variable displacement pump containing two vertical bores 12, 14, which are offset from the pump center line. These bores contain the mechanisms for changing the pump displacement, as will be hereinafter described. The pump body consists of a head 16 seated upon a housing 18. Contained within the head is an inlet passage 20 and an outlet passage 22. Also shown is a section of the high-pressure conduit 24 through which the pump flow discharges.

Referring now to FIG. 2, there is shown a vertical, cross-sectional view of variable displacement pump 10 taken along line II-II in FIG. 1. A pump cylinder barrel 26, having a plurality of axially-oriented bores 28 and a plurality of reciprocably-mounted pistons therein, is located within the pump housing. The pistons are guided by a slipper pad 30 rotatably, slidably mounted on support means in the form of a nonrotatable but tiltable swash plate 32. The swash plate is pivotable about a transverse axis by means of transverse pivot pins (not shown) retained in transverse bores (not shown) in the sides of housing 18.

The angle of tilt of the swash plate 32 with respect to the axial direction of the bores 28 determines the amount of stroke or displacement of the pistons 34 within their respective bores 28 in the conventional manner. Since slipper pad 30 is ball-and-socket-joint connected to pistons 34 which, in turn, ride on surface 36 of the swash plate, it can be seen that if the swash plate were rotated clockwise about its pivot point from the position shown, the reciprocating action of the pistons in their respective bores would be minimized. Thus, a minimum displacement of fluid would'occur through outlet passage 22 by way of kidney port 38.

The operation of the invention is as follows. Prior to start-up of the driving engine, the swash plate 32 will assume the attitude shown in FIG. 2. It will assume this position due to the biasing force of a spring 40, the biasing force of which acts through a spring retainer 42 and the mostly internal sleeve 44 of a cartridge assembly or servomechanism shown generally at 46. The internal workings of this cartridge assembly will be hereinafter described. It is sufficient to state that the internal mechanism (not shown) causes a piston 48 to force an arm extension 50 of swash plate 32 against which it bears to, in turn, be in contact with a maximum displacement stop or bolt 52. The maximum displacement stop may be conveniently threaded into the housing 18 by thread means (not shown) to provide adjustability.

When the driving engine is started, the pump cylinder barrel 26 starts to rotate by means of shaft 53 connected to the driving engine on other power means (not shown) and an output fluid flow will be generated along with an output fluid pressure. This can be communicated to the various implement circuits (not shown) through appropriate control valves therein. If the control valves for those particular circuits are of the closed center design, then they must be protected by relief valve means which will divert pump flow back to tank after a predetermined maximum pressure is reached.

However, prior to reaching that maximum of pressure, and as best seen in FIG. 1, fluid flow is directed to a valve 54 by way of a line 56 intercommunicating with high-pressure conduit 24. When all of the implement control valves (not shown) are in neutral, the valve 54 will be pilot operated through a line 58 such that fluid pressure from line 56 will be directed to a line 60 while the line 62, the purpose of which will be hereinafter described, is connected to tank by way of a line 64. The hydraulic circuitry just described can be further protected by means of a relief valve 66 so as to limit the maximum pressure of the output of pump 10 when the main control valves, and subsequently valve 54, are in a neutral position.

As best seen in FIG. 2, fluid pressure from line 60 is transmitted to a chamber 68 in cartridge assembly 70 by way of a passage 72, an annulus 74, and passages 76 and 78. The fluid pressure thus transmitted will exert a force on an end of piston 80 projecting from a bore in the end of cartridge assembly 70. Piston 80 will force an arm extension 82 of swash plate 32 downwardly against a minimum displacement stop 84. This stop may be conveniently in the form of a threaded stud having a nut thereon which stud may be adjustably attached to a threaded bore (not shown) in the housing 18. When the swash plate is in this position, a minimum displacement of fluid will occur.

Turning again to FIG. 1, shifting of one of the main control valves so as to actuate an implement circuit causes valve 54 to be pilot operated by means of line 58. This blocks intercommunication between lines 62 and 64 and additionally intercommunicates lines 56, 60 and 62 so that the variable cutoff mechanism can be actuated, as will be hereinafter described. As may be seen in FIG. 2, fluid thus communicated is directed to an annulus 86 by means of intersecting passages 88 and 90 leading from line 62.

Referring now to FIG. 3, fluid flow from annulus 86 is then communicated through a passage 92 and thence to an annulus 94 in cartridge assembly 46 which further comprises a cartridge housing 96. Annulus 94 communicates with a plurality of passages 98 contained in sleeve 44. Passages 98 are further in fluid communication with an annular groove 100 peripherally located in servospool 102 which is contained within sleeve 44. Annulus 86 further fluidly communicates with a passage 104 in the housing which leads to a poppet valve 106 contained within housing 108, as best seen in FIG. 2. The purpose of this valve will be explained hereinbelow.

Returning to FIG. 3, fluid is further directed from annular groove 100 to an annulus 110 by way of a plurality of passages 112, the latter being contained within sleeve 44. From annulus 110, fluid is directed by way of a longitudinally-oriented passage 114 to a chamber 116 in the end of the cartridge housing 96.

Since the force from pressure in chamber 116 and the force spring 40 acting on piston 48, which force is transmitted by sleeve 44, end plate 118, and piston extension 120, is greater than the countervailing force from pressure in chamber 68 (FIG. 2) acting upon pis ton 80, the swash plate 32 will be swivelled to its maximum displacement position, as determined by stop means 52. This coincides with point B on the graph of FIG. 4. This is in contrast to the initial position at startup when a swash plate 32 is swivelled against stop means 52 and in which the pressure and flow are equivalent to that denoted by point A in the graph of the same figure.

When the pressure reaches point B, as best seen in FIG. 2, the force thereof will be great enough to move a piston 122 upwardly against the biasing force of a spring 124 until the passage 78 is in metered communication with passage 126. Passage 126, in turn, communicates fluid into an annulus 128 by means of a passage 130. Pressure is communicated from passage 130 into chamber 131 where it assists spring 124 in the downward actuation of piston 132, to provide the reducing valve function.

As seen in FIG. 4, as the load on the implement circuit increases, the pressure of the pump flow will also increase along line B-C. It has been found that an area ratio of the areas of piston 122 and piston 132 should be selected to provide a reduction range between 5 to l and to 1 reduction of pressure in passage 78 will be available to passage 134 which is also in communication with annulus 128. Passage l34.is also in communication with an annulus 136 by way of passage 138 and annulus 140 formed by plug 142 threadably secured in the top surface 144 of head 16. Annulus 136 is also in communication with spring chamber 146 of the pilot valve housing 108 by means of passage 148.

Turning to FIG. 3, annulus 136 is also in communication with an annulus 150 in the cartridge housing 96 by means of a plurality of passages 152. Annulus 150, in turn, communicates with a chamber 154. The force of pressure in chamber 154 is thus enabled to work against a differential area of the servo-spool 102, which area is shown generally at 156.

When the pressure in the chamber 154 reaches a sufficiently high value, which value will be in direct proportion, at the aforementioned ratio, to the pressure shown at point C on the graph of FIG. 2, the servospool 102 will start to move upwardly against the biasing force of spring 158, which spring is concentrically located within spring 40. The spool will continue to move until a balance of forces on the spool reaches equilibrium. In so doing, the system pressure in passage 98 will be blocked from communication with annulus groove 100, which groove is still in communication with annulus 110 by way of passages 112.

Annulus 160 is in communication with tank pressure by way of passages 162 and 164, chamber 166, a plurality of passages 168 in end plate 118, a chamber 170,

and passage 172. This concurrently allows chamber (FIG. 2) will be able to overcome the summation of forces acting on piston 48.

This will result in the movement of servosleeve 44 upwardly against the biasing force of spring 40 by means of the force communicated through piston extension and end plate 118. The movement thus described will continue until sleeve 44 catches up to and achieves the identical amount of upward movement as spool 102. The sleeve will then modulate at the position, as is conventional in the operation of a linear servovalve assembly. This action will allow the swash plate 32 to be swivelled toward minimum displacement position. This swivelling will be variable so as to produce a pump flow/pressure characteristic as shown along line C-D of FIG. 4. As may be seen from the subject figure, as system pressure increases, the pump flow will linearly decrease, thus maintaining horsepower at a constant level. Variable cutoff of pump flow is thus provided until point D is reached. When system pressure reaches point D, the pressure is communicated through passage 104 to unseat poppet valve 106 against the bias of spring 174, as best seen in FIG. 2. The strength of spring constant 174 can, of course, be set in the conventional manner. Pressure in passage 104 will thence be communicated into chamber 146 and thence to annulus 136 by way of passage 148.

As best seen in FIG. 3, fluid will then be communicated into annulus by way of passages 152. This will cause pressure to act directly upon the differential area 156 which can, in turn, move both the servo-spool 102 and subsequently sleeve 44 (mechanically by piston extension 120) far enough to allow swash plate assembly to swivel all the way against a minimum displacement stop. This rapid movement will cause sharp cutoff of pump discharge to point E on the graph of FIG. 4, along line DE. Thus, sharp cutoff of pump discharge is provided.

Point E denotes the point at the minimum flow and maximum pressure that will be maintained in the implement circuit being actuated. The shaded area within triangle CDF represents horse power savings which accrue to the instant invention over a device in which only sharp cutoff is provided, which sharp cutoff occurs along the line F-D-E. With the variable cutoff means of this invention, speed of actuation of the implements decreases at the same rate that pump flow is decreased, whereas in a purely sharp cutoff system, the implement actuation does not decrease until near maximum pressure is reached (point F).

Turning now to FIG. 5, there is shown a partial crosssectional view of an alternate embodiment of the instant invention. In the following discussion, single primes are used in conjunction with elements finding their analogous counterpart in the primary embodiment. The instant embodiment provides for the use of a separate external pressure source to actuate the servovalve cartridge assembly.

In this situation, external pressure is provided by means of a pressure source in the form of an underspeed valve 176, such as that which is more fully disclosed in copending Application Ser. No. 68,315 filed Aug. 31, 1970, and now US. Pat. No. 3,727,628, issued Apr. 17, 1973. The description of the underspeed valve contained therein is hereby'expressly incorporated by reference.

. The operation of the subject embodiment is as follows. Fluid pressure directed from underspeed valve 176 by way of a line 178 enters a plug 180 by means of passage 182. Contained within a chamber 184 of plug 180 is a spring-biased ball check valve 186.

This check valve prevents excessive pressure buildup communicating back to the underspeed valve whenever pump output pressure available in passages 90, 188 and 104' opens poppet valve 106, as explained in the primary embodiment.

Turning again TO FIG. 2, it may be noted that pump rotation will be in one direction only due to the fact that the swash plate can be swivelled only in a clockwise direction. Therefore, when pump flow direction is to be changed, the capsule or cartridge assemblies must also be interchanged along with interchanging of stops 52 and 84 in order that the inlet and outlet ports will remain the same. This interchanging may be easily accomplished with the device of the instant invention since cartridge assemblies 46 and 70 have the same exterior circumferential dimensions as well as being generally cylindrical. In addition, and to facilitate this interchanging, the locations and dimensions of the leftmost annuli 86, 136, duplicate those of the rightmost annuli 74, 128, respectively. More particularly, annulus 136 is located at the same vertical level in the head as annulus 128 wherein interchanging of assemblies 46 and 70 results in passage 152 in cartridge assembly 46 being in fluid communication with passage 148 leading to chamber 146 by way of the annuli and passages 134, 136, and annulus 140.

Similarly, annuli 74 and 86 are located at the same vertical level in the head. Passage 90 intercommunicates the two annuli whereby passage 92 in cartridge assembly 46 may be communicated with passage 104 and thence to poppet valve 106.

To interchange the assemblies, head 16 is removed from the housing 18 by detaching fastening means (not shown). Assembly 46 is removed from its bore in head 16 and cap member 190 is backed out of the bore to which it is threadably secured. Similarly, threaded plug 192 is backed out of its bore in which it is also threadably secured. It should be noted at this point that flange means 194, 196 are located intermediate the ends of cartridge assemblies 46, 70, respectively, which flange means cooperate with correspondingly-shaped openings in the head. These openings are provided at the interface between the head and the housing and serve to structurally retain the respective cartridge assemblies. The respective cartridge assemblies are interchanged and resecured by reversing the above steps. In addition, stop means 52 and 84 are interchanged. The head 16 is then resecured to the housing 18.

Referring now to FIG. 6, there is shown a partial cross-sectional view of yet another alternate embodiment of the instant invention. In the following discussion, double primes are used in conjunction with elements finding their analogous counterpart in the primary embodiment. The instant embodiment provides a mechanical link to an external piston means to'actuate the servovalve cartridge assembly. V

In the instant embodiment, a spring biased piston means 198 (shown in cross section) is mechanically linked to cartridge assembly or servomechanism 46" to control the pump displacement. The mechanical link is accomplished by means of linkage rod 200 which slidably extends through a hole in cap member 190", an end of which is threadably connected to spool 102". With this embodiment, annulus 128" is intercommunicated with piston valve 198 by means of line 202. This is in contrast to that shown in FIG. 2 for the primary embodiment wherein intercommunication is with cartridge assembly 46 by way of passages 134, 138 and annulus 140. Line 202 is connected to either of the inlet port of the valve 198.

There are some applications where it is desirable to use pressure from an external pressure source to cause servovalve 46" to operate in lieu of the mechanical link system just described. In this case, pressure is channeled to the servovalve from an external pressure source (not shown) by way of line 204 and passage 148". With this system, passage 206 will be blocked and the mechanical linkage will not be used.

It is to be understood that the foregoing description is merely illustrative of preferred embodiments of the invention, and that the scope of the invention is not to be limited thereto, but is to be determined by the scope of the appended claims.

What is claimed is:

1. A variable displacement axial piston pump of the type having a housing containing an inlet port and a discharge port, a plurality of piston means and an angularly-adjustable swash plate means for determining piston displacement and thereby the amount of fluid discharged from the pump, resilient means associated with a first cartridge assembly means biasing the swash plate means to its maximum discharge position whereby a maximum fluid is displaced by the pump and pressure responsive means in a second cartridge assembly means for shifting said swash plate means to its minimum discharge position in response to a means generating a pilot pressure, and reversing means whereby interchanging said first cartridge assembly means with said second cartridge assembly means causes the pump flow direction to be reversed, said reversing means comprising a pair of stop means, one of which is associated with the swash plate adjacent said first cartridge assembly means and the other of which is associated with the other of said cartridge assembly means, said stop means being unconnected with and freely selectively adjustable with respect to said swash plate means, means whereby said stop means may be interchanged in conformance with the interchanging of the first with the second cartridge assembly forming first chamber means containing both a motor piston and valve means, said assembly means, wherein said first cartridge assembly means comprises a first cartridge member in a first bore in said housing means, said second cartridge assembly forming second chamber means containing a motor piston, said assembly means comprises a second cartridge member in a second bore in said housing, wherein said first and second cartridge members are dimensioned so as to be interchangeable within said bores and further including a plurality of spaced annuli in said first bore and a plurality of spaced annuli in said second bore and wherein each of the annuli in said first bore has a counterpart annuli in said second bore in the same vertical level in said head, said valve means operative to direct pilot fluid to one of said first and second chamber means.

2. The pump of claim 1 wherein said second cartridge member comprises a generally elongated member and said pressure responsive means comprises a first reciprocable piston extending froma bore in one end of said cartridge member and thereby defining a first chamber therein.

3. The pump of claim 2 wherein said cartridge member is a generally elongated member and wherein said resilient means in said first cartridge member comprises a second reciprocable piston extending from a reciprocable bore in one end of said cartridge member and thereby defining a second chamber therein.

4. The pump of claim 3 further including a pilot line in said housing adapted to sense pilot pressure from a hydraulic system and wherein said means for shifting said swash plate to its minimum discharge position comprises spool means responsive to pilot line pressure for directing pressure fluid to said first chamber so as to cause said first reciprocable piston to tilt said swash plate means to the minimum discharge position.

5. The pump of claim 1 wherein the plurality of spaced annuli comprises two in each of said bores and further including passage means intercommunicating one annulus in said first bore with its counterpart in said second bore.

6. The pump of claim 1 wherein said means generating a pilot pressure comprises means adapted to communicate said pump with an external fluid pressure source.

7. The pump of claim 6 wherein said means generating a pilot pressure is contained in said second cartridge assembly means.

8. The pump of claim 7 further including passage means in said pump housing means for fluidly intercommunicating said means generating a pilot pressure with said first cartridge means internally of said housing means.

9. A variable displacement axial piston pump of the type having a housing containing an inlet port and a discharge port, a plurality of piston means and an angularly-adjustable swash plate means for determining piston displacement and thereby the amount of fluid discharged from the pump, resilient means associated with a first cartridge assembly means biasing the swash plate means to its maximum discharge position whereby a maximum of fluid is displaced by the pump and pressure responsive means in a second cartridge assembly means for shifting said swash plate means to its minimum discharge position in response to a means generating a pilot pressure, wherein said means generating a pilot pressure comprises means adapted to intercommunicate said second cartridge assembly means with the pressure fluid discharged from the pump, and pilot valve means adapted to be responsive to hydraulic system pressure for selectively intercommunicating the pressure fluid discharged with said second cartridge means, and wherein said pump further comprises means intercommunicating said first cartridge assembly means with said second cartridge assembly means internally of said housing, and means in said pilot valve means responsive to hydraulic system pressure for intercommunicating said means intercommunicating said cartridge means to tank coincident with movement of the pilot valve means.

10. A variable displacement axial piston pump of the type having a housing containing an inlet port and a discharge port, a plurality of piston means and an angularly-adjustable swash plate means for determining piston displacement and thereby the amount of fluid discharged from the pump, resilient means associated with a first cartridge assembly means biasing the swash plate means to its maximum discharge position whereby a maximum of fluid is displaced by the pump and pressure responsive means in a second cartridge assembly means for shifting said swash plate means to its minimum discharge position in response to a means generating a pilot pressure,

wherein said first cartridge assembly means includes mechanical linkage means adapted for movement by said means for generating a pilot pressure and a valve spool, and wherein said mechanical linkage means is fixed to an end of said spool and extends from and externally to said housing, and further including piston means intermediate said means generating a pilot pressure and said linkage means for translating pressure into mechanical movement and thereby to actuate said valve spool. 

1. A variable displacement axial piston pump of the type having a housing containing an inlet port and a discharge port, a plurality of piston means and an angularly-adjustable swash plate means for determining piston displacement and thereby the amount of fluid discharged from the pump, resilient means associated with a first cartridge assembly means biasing the swash plate means to its maximum discharge position whereby a maximum fluid is displaced by the pump and pressure responsive means in a second cartridge assembly means for shifting said swash plate means to its minimum discharge position in response to a means generating a pilot pressure, and reversing means whereby interchanging said first cartridge assembly means with said second cartridge assembly means causes the pump flow direction to be reversed, said reversing means comprising a pair of stop means, one of which is associated with the swash plate adjacent said first cartridge assembly means and the other of which is associated with the other of said cartridge assembly means, said stop means being unconnected with and freely selectively adjustable with respect to said swash plate means, means whereby said stop means may be interchanged in conformance with the interchanging of the first with the second cartridge assembly forming first chamber means containing both a motor piston and valve means, said assembly means, wherein said first cartridge assembly means comprises a first cartridge member in a first bore in said housing means, said second cartridge assembly forming second chamber means containing a motor piston, said assembly means comprises a second cartridge member in a second bore in said housing, wherein said first and second cartridge members are dimensioned so as to be interchangeable within said bores and further including a plurality of spaced annuli in said first bore and a plurality of spaced annuli in said second bore and wherein each of the annuli in said first bore has a counterpart annuli in said second bore in the same vertical level in sAid head, said valve means operative to direct pilot fluid to one of said first and second chamber means.
 2. The pump of claim 1 wherein said second cartridge member comprises a generally elongated member and said pressure responsive means comprises a first reciprocable piston extending from a bore in one end of said cartridge member and thereby defining a first chamber therein.
 3. The pump of claim 2 wherein said cartridge member is a generally elongated member and wherein said resilient means in said first cartridge member comprises a second reciprocable piston extending from a reciprocable bore in one end of said cartridge member and thereby defining a second chamber therein.
 4. The pump of claim 3 further including a pilot line in said housing adapted to sense pilot pressure from a hydraulic system and wherein said means for shifting said swash plate to its minimum discharge position comprises spool means responsive to pilot line pressure for directing pressure fluid to said first chamber so as to cause said first reciprocable piston to tilt said swash plate means to the minimum discharge position.
 5. The pump of claim 1 wherein the plurality of spaced annuli comprises two in each of said bores and further including passage means intercommunicating one annulus in said first bore with its counterpart in said second bore.
 6. The pump of claim 1 wherein said means generating a pilot pressure comprises means adapted to communicate said pump with an external fluid pressure source.
 7. The pump of claim 6 wherein said means generating a pilot pressure is contained in said second cartridge assembly means.
 8. The pump of claim 7 further including passage means in said pump housing means for fluidly intercommunicating said means generating a pilot pressure with said first cartridge means internally of said housing means.
 9. A variable displacement axial piston pump of the type having a housing containing an inlet port and a discharge port, a plurality of piston means and an angularly-adjustable swash plate means for determining piston displacement and thereby the amount of fluid discharged from the pump, resilient means associated with a first cartridge assembly means biasing the swash plate means to its maximum discharge position whereby a maximum of fluid is displaced by the pump and pressure responsive means in a second cartridge assembly means for shifting said swash plate means to its minimum discharge position in response to a means generating a pilot pressure, wherein said means generating a pilot pressure comprises means adapted to intercommunicate said second cartridge assembly means with the pressure fluid discharged from the pump, and pilot valve means adapted to be responsive to hydraulic system pressure for selectively intercommunicating the pressure fluid discharged with said second cartridge means, and wherein said pump further comprises means intercommunicating said first cartridge assembly means with said second cartridge assembly means internally of said housing, and means in said pilot valve means responsive to hydraulic system pressure for intercommunicating said means intercommunicating said cartridge means to tank coincident with movement of the pilot valve means.
 10. A variable displacement axial piston pump of the type having a housing containing an inlet port and a discharge port, a plurality of piston means and an angularly-adjustable swash plate means for determining piston displacement and thereby the amount of fluid discharged from the pump, resilient means associated with a first cartridge assembly means biasing the swash plate means to its maximum discharge position whereby a maximum of fluid is displaced by the pump and pressure responsive means in a second cartridge assembly means for shifting said swash plate means to its minimum discharge position in response to a means generating a pilot pressure, wherein said first cartridge assembly means includes mechanical linkage means adapted for movement by saId means for generating a pilot pressure and a valve spool, and wherein said mechanical linkage means is fixed to an end of said spool and extends from and externally to said housing, and further including piston means intermediate said means generating a pilot pressure and said linkage means for translating pressure into mechanical movement and thereby to actuate said valve spool. 